Multiple firing heads have been used on a single well bore perforating gun for the purpose of providing redundancy in the event of one firing head failing to operate properly. Various configurations have been used to connect the multiple firing heads to the single gun, including the placement of a single firing head on each end of the perforating gun. Another configuration involves the placement of two firing heads, in line, on one end of the perforating gun between the gun and the tubing. The two firing heads are typically operable by different means, for example, tubing pressure, annulus pressure, hydraulic control lines, and mechanical means such as a "go devil". Various differential pressure schemes have also been utilized.
The use of redundant top-fired heads for tubing-conveyed perforating guns often requires the splicing of detonation cords and the routing of one detonation cord from an upper firing head around a lower head through an annular space or otherwise radially offset from the center of the string. The types of detonation transfers which are required and the round-about routing of a detonation cord results in significant unreliability of the redundant system, thereby limiting its usefulness.